The present invention relates to submersible vehicles and in particular, to remotely controlled submersible vehicles (referred to hereafter as RCSVs). Such vehicles are also commonly referred to as remotely operated vehicles (ROVs).
RCSVs are suspended, in use, from an umbilical cord through which various services are provided, including control signals for controlling the RCSV. RCSVs may also be used to transport payloads from one location to another. Conventional RCSVs, are designed to carry tooling packages attached around the periphery of the RCSV, particularly the fore, aft, side or underneath faces of the RCSV. This can result in poor performance and poor controllability of the RCSV as discussed below. RCSVs are commonly used to perform tasks at subsea oil installations such as wellheads and manifolds. These tasks may require specialist tools or equipment. If so, the tools or equipment may be carried on board the RCSV and operated through the control system onboard the RCSV, controlled from the sea surface by means of the umbilical cord.
Typically, RCSVs are propelled by a number of hydraulically or electrically driven thrusters (sometimes called propulsors) attached to the frame of the RCSV, to point in various directions, primarily generally vertically or horizontally. It is desirable to maintain an unimpaired flow path (commonly called the wash path) of sea water into and out of the thruster in order to maximise the motive power provided to the RCSV, and thereby optimise performance. In order to reduce impairment in the wash path by components of the RCSV, it has been proposed to mount thrusters at an angle to the main axis of the RCSV, at or near the corners of the RCSV. However when a tooling package is mounted at a position at the periphery of the RCSV, its presence is likely to impair the wash path of one or more of the thrusters, which can impede the performance and controllability of the RCSV. Alternatively, the size or shape of a payload which can be deployed may be limited in order to avoid impairment of a wash path.
A further problem with previous proposals arises because the effect of the additional weight of the payload attached at a position around the periphery of the RCSV is to move the centre of gravity of the combination away from the position of the centre of gravity of the RCSV alone. However, the position at which lifting gear is attached to the RCSV would normally be chosen to be above or close to the centre of gravity of the RCSV in order to maintain the RCSV substantially horizontal when being lifted and handled during deployment and recovery. The presence of the payload will therefore deflect the RCSV from this horizontal attitude, resulting in increased lifting and handling problems during deployment and recovery.